This application claims the benefit of Korean Application No. 2001-62535, filed Oct. 11, 2001, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a paper feeding device for a printer. More specifically, the present invention relates to a paper feeding device for a printer, in which an automatic compensation unit is provided.
2. Description of the Related Art
Generally, a printer is provided with a paper feeding device which is secured on the printer body, for feeding the paper sheets. The printer paper feeding device feeds paper sheets from a paper feeding cassette one by one into a printer body in accordance with printing signals. The paper feeding is achieved by exerting a vertical force on a rubber roller so as to generate a friction force between the paper sheet and the roller.
However, as the paper sheets are fed into the printer body and thus the stack of paper becomes lower, the vertical force varies, thereby varying the friction force as well. This hinders smooth paper feeding, thus the variation of the vertical force must remain within a certain range.
FIG. 1 schematically illustrates the construction of the conventional printer paper feeding device in which an automatic compensation unit is provided to compensate for the vertical forces. FIG. 2 illustrates variations of paper contact angles of the paper feeding device of FIG. 1. That is, FIG. 2 illustrates an angle between an uppermost paper sheet of the paper stack at maximum height and the automatic compensation unit, and an angle between the lowermost paper sheet and the automatic compensation unit. Referring to FIGS. 1 and 2, the paper contact angles are varied from an angle xcex21 (when the paper stack is at maximum height) to an angle xcex22 (when only the last paper is left).
As shown in FIG. 1, the printer paper feeding device includes a pickup shaft 11 for transmitting the rotation torque of a driving source (not illustrated), an automatic compensation unit 10 provided with a pickup roller 15, a paper feeding cassette 20 for accommodating a paper stack 30, and a separating wall 23 formed on one end of the paper feeding cassette 20 in a paper-feeding direction, for separating the paper sheets.
The automatic compensation unit 10 comprises a train of four gears 13a, 13b, 13c and 13d. The train of four gears 13a, 13b, 13c and 13d are pivotally connected to the pickup shaft 11 so that the first gear 13a can transmit the rotation torque T of the pickup shaft 11 to the pickup roller 15, and the pickup roller 15 can vary its contact position on the paper stack 30 as the height of the paper stack 30 is decreased during the printing operation. The pickup roller 15 is coupled coaxially to a shaft of the 4th gear 13d by being interlocked to the pickup shaft 11.
The operation of the printer paper feeding device will now be described. When the pickup shaft 11 is rotated by the driving source (not illustrated), then the first gear 13a rotates, and the second and third gears 13b and 13c rotate so as to ultimately transmit the power to the fourth gear 13d. The pickup roller 15 is assembled to the shaft of the fourth gear 13d, and therefore, if the fourth gear 13d rotates, then the pickup roller 15 also rotates. If the pickup roller 15 rotates, the uppermost sheets of paper of the cassette 20 are biased forward due to the friction force between the pickup roller 15 and the paper stack 30. Then, due to the presence of the separating wall 23, only the uppermost sheet of paper is separated and fed into the printer body.
If the paper sheets are to be separated one by one, the following conditions must be satisfied:
Fpick greater than Ffric greater than Fd greater than Fdouble xe2x80x83xe2x80x83 less than Formula 1 greater than 
where Fpick is the feeding force due to the rotation torque of the pickup roller 15, Ffric is the carrying force due to the friction between the pickup roller 15 and the paper stack 30, Fd is the resistant force acting on the leading edge of the paper by the separating wall 23 and Fdouble is the carrying force for the second sheet paper next to the uppermost paper sheet.
First, Fpick is calculated as follows:
Fpick=T/r xe2x80x83xe2x80x83 less than Formula 2 greater than 
where T is the rotation torque of the pickup shaft 11 and r is the radius of the pickup roller 15, Ffric is calculated as follows:
Ffric=xcexcrollxc3x97Ntotal xe2x80x83xe2x80x83 less than Formula 3 greater than 
where xcexcroll is the friction coefficient between the paper stack 30 and the pickup roller 15 and Ntotal is the maximum vertical force pressing on the paper stack 30 by the pickup roller 15.
Finally, Fdouble is calculated as follows:
Fdouble=xcexcpaperxc3x97Ntotal xe2x80x83xe2x80x83 less than Formula 4 greater than 
where xcexcpaper is the friction coefficient between the paper sheets, and Ntotal is the maximum vertical force pressing on the paper stack 30 by the pickup roller 15.
As shown in Formulas 2 through 4, if factors such as the rotation torque T of the pickup shaft 11, the radius r of the pickup roller 15, the separating wall 23 and the type of paper sheet are properly chosen, then Fpick and Fd become constant regardless of a height h of the paper stack 30, and therefore, the height h is constant. However, Ffric and Fdouble vary in accordance with the height of the paper stack 30, and therefore, Ffric and Fdouble are treated as variables. Accordingly, whether Formula 1 is satisfied or not is determined by the value of Ntotal.
Ntotal is the vertical force pressing on the paper stack 30 by the pickup roller 15, and therefore, it can be expressed as the vertical force acting on the pickup roller 15. Ntotal is the sum total of: a vertical force NR due to the rotation torque of the pickup roller 15, a vertical force NA due to a link 12 of the automatic compensation unit 10, and a vertical force NW due to the weight of the automatic compensation unit 10.
Ntotal=NR+NA+NW xe2x80x83xe2x80x83 less than Formula 5 greater than 
In the above formula, the vertical force NR acts such that the rotation torque of the pickup roller 15 increases the vertical force NR at the instant when Fd greater than Ffric so as to stop the feeding of the paper sheets. Referring to FIG. 3A, a maximum value of the vertical force NR is calculated by the following formula.                               N          R                =                                            T              r                        ·            cos                    ⁢                      xe2x80x83                    ⁢                      β            ·            sin                    ⁢                      xe2x80x83                    ⁢          β                                     less than Formula  6 greater than             
where T is the rotation torque of the pickup roller 15, r is the radius of the pickup roller 15, and xcex2 is the paper contact angle.
Further, the vertical force NA due to the action of the link 12 of the automatic compensation unit is generated when the carrying force Ffric due to the pickup roller 15 attains equilibrium with the paper feed resistance Fd to stop the rotation of the pickup roller 15. A maximum value of the vertical force NA is calculated based on the following formula by referring to FIG. 3B.                               N          A                =                                            T              L                        ·            cos                    ⁢                      xe2x80x83                    ⁢          β                                     less than Formula  7 greater than             
where L is the length of the link 12 of the automatic compensation unit 10, T is the rotation torque of the pickup roller 15, and xcex2 is the paper contact angle.
The vertical force NW due to the weight of the automatic compensation unit 10 is calculated based on the following formula by referring to FIG. 3C.                               N          W                =                  W          ·                      D            L                                               less than Formula  8 greater than             
where W is the total weight of the automatic compensation unit 10, D is the distance from the center of the first gear 13a to the center of gravity of the automatic compensation unit 10, and L is the length of the link 12 of the automatic compensation unit 10.
Accordingly, if Formulas 6 through 8 are substituted into Formula 5, then Formula 5 can be expressed as follows:                                           N            total                    =                                                                      T                  r                                ·                sin                            ⁢                              xe2x80x83                            ⁢                              β                ·                                  xe2x80x83                                ⁢                cos                            ⁢                              xe2x80x83                            ⁢              β                        +                          T              L                                      ⁣                                            ·              cos                        ⁢                          xe2x80x83                        ⁢            β                    +                      W            ·                          D              L                                                           less than Formula  9 greater than             
Ntotal is the maximum vertical force acting on the pickup roller 15 during the generation of the feed resistance Fd, and this force acts until the conditions of Formula 1 are satisfied. However, in the normal paper feeding operation, the paper sheet advances before the vertical force acts. If the carrying force Ffric does not exceed the paper feed resistance Fd, then NR, and NA automatically and gradually increase the vertical force Ntotal. Thus, if the vertical force increases, the carrying force Ffric due to friction increases according to Formula 3, with the result that the conditions of Formula 1 are satisfied, thereby allowing the paper sheet to advance.
If the ratio of the radius r of the pickup roller 15 to the length L of the link 12 is 1:5, based on Formula 9, then the relationship between the paper contact angle xcex2 and the vertical force Ntotal is illustrated in FIG. 4. The maximum value is seen near a xcex2 value of 45 degrees.
If the uppermost paper sheet is to be fed, a proper force between the carrying force Ffric of the first paper and the forward biasing force Fdouble of the second paper must be selected such that the resistant force Fd would be a factor. However, as the paper is fed and thereby gradually the height h of the paper stack 30 lowers, then the paper contact angle xcex2 is gradually varied. Specifically, as shown in FIG. 2, the paper contact angle xcex2 varies from the angle xcex21 to the angle xcex22.
A variation amount xcex94xcex8 (xcex22-xcex21) of the paper contact angle is proportional to: (1) the paper stacking height h; (2) the length L of the link 12; and (3) the initial paper contact angle xcex2 1 or xcex2 2.
Referring to FIG. 2, when xcex2 2 is varied from 0xc2x0 to 90xc2x0, the variation amount xcex94xcex8 is greatly varied. Specifically, from             sin              -        1              ⁡          (              h        L            )        ⁢      xe2x80x83    ⁢  to  ⁢      xe2x80x83    ⁢                    cos                  -          1                    ⁡              (                              L            -            h                    L                )              .  
In order to avoid such a large variation, xcex22 is generally between 7xc2x0 and 15xc2x0,
However, within this paper contact angle range, a steep variation of the vertical force Ntotal occurs between xcex2 1 and xcex2 2, as shown in the graph of FIG. 4. If the maximum amount of paper is loaded in the paper cassette 20, a great difference in the vertical force Ntotal occurs between the first paper and the last paper. Therefore, instances in which Formula 1 cannot be satisfied are likely. Specifically, when the variation between Ffric and Fdouble cannot satisfy Formula 1, a feed failure or a double feed occurs.
Furthermore, the paper feed resistance Fd is different depending on the type and the stiffness of the paper. Therefore, if all types of paper are to satisfy Formula 1, then the variation range between Ffric and Fdouble must be as small as possible.
Accordingly, it is an object of the present invention to overcome the above described disadvantages of the conventional techniques.
Accordingly, it is another object of the present invention to provide a paper feeding device for a printer, in which a variation amount of a vertical force is kept to a minimum so as to prevent feeding errors, even when using various kinds of printing media.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
The foregoing and other objects of the present invention are achieved by providing a paper feeding device for a printer including a paper feeding cassette to load a plurality of paper sheets; a driving power source; a driving gear driven by the driving power source; a passive gear rotated interlockingly with the driving gear; a first link having a first end pivotally installed on a rotation shaft of the driving gear, and a second end coupled to a rotation shaft of the passive gear; a pickup gear rotated interlockingly with the passive gear; a second link having a first end rotatably installed on the rotation shaft of the passive gear, and a second end coupled to a rotation shaft of the pickup gear; a pickup roller coaxially coupled to the pickup gear, to simultaneously rotate and press the paper sheets so as to feed the sheets one by one into a printer body; and a supporting arm with a first end coupled to a rotation shaft of the pickup roller, and with a second end pivotally installed on a side of the printer body.
Furthermore, a connecting gear is disposed between the driving gear and the passive gear, to transmit a rotation torque of the driving gear to the passive gear and an idler gear is disposed between the passive gear and the pickup gear, to transmit a rotation torque of the passive gear to the pickup gear.
Furthermore, the pickup gear, the connecting gears, the passive gear, the idler gear and the pickup gear have the same shape.
Furthermore, there is included a separating wall installed on an end of the paper feeding cassette, to contact a leading edge of the paper sheets and wherein the separating wall includes a top portion inclined in a paper feeding direction.
In the paper feeding device of the present invention as described above, the paper contact angle is minimized even when the paper sheets are continuously fed, thereby lowering the height of the paper stack. Thus, the variation of the vertical force acting on the pickup roller is minimized, thereby preventing paper-feeding errors, even in the case where various kinds of paper are used.